Magnetic resonance imaging (MRI) is a clinical diagnostic and research procedure that uses a high-strength magnet and radio-frequency signals to produce images. The most abundant molecular species in biological tissues is water. It is the quantum mechanical "spin" of the water proton nuclei that ultimately gives rise to the signal in imaging experiments. In MRI the sample to be imaged is placed in a strong static magnetic field (1-12 Tesla) and the spins are excited with a pulse of radio frequency (RF) radiation to produce a net magnetization in the sample. Various magnetic field gradients and other RF pulses then act on the spins to code spatial information into the recorded signals. MRI is able to generate structural information in three dimensions in relatively short time spans.
There is rapidly growing body of literature demonstrating the clinical effectiveness of paramagnetic contrast agents; currently there are at least eight different contrast agents in clinical trials or in use. These agents provide further contrast, and thus enhanced images, wherever the contrast agent is found. For example, the approved contrast agents outlined below may be injected into the circulatory system and used to visualize vascular structures and abnormalities, amongst other things. The capacity to differentiate regions or tissues that may be magnetically similar but histologically different is a major impetus for the preparation of these agents.
The lanthanide atom Gd(III), has generally been chosen as the metal atom for contrast agents because it has a high magnetic moment (.mu..sup.2 =63BM.sup.2), a symmetric electronic ground state, (S.sup.8), the largest paramagnetic dipole and the greatest paramagnetic relaxivity of any element. Gd(III) is rendered nontoxic by chelation. To date, a number of chelators have been used, including diethylenetriaminepentaacetic (DTPA), 1,4,7,10-tetraazacyclododecane'-N,N'N",N"'-tetracetic acid (DOTA), and derivatives thereof. See U.S. Pat. Nos. 5,155,215, 5,087,440, 5,219,553, 5,188,816, 4,885,363, 5,358,704, 5,262,532, and Meyer et al., Invest. Radiol. 25: S53 (1990).
The stability constant (K) for Gd(DTPA) is very high (logK=22.4) and is more commonly known as the formation constant (the higher the logK, the more stable the complex). This thermodynamic parameter indicates the fraction of Gd(III) ions that are in the unbound state will be quite small and should not be confused with the rate (kinetic stability) at which the loss of metal occurs. The water soluble Gd(DTPA)-chelate is stable, nontoxic, and one of the most widely used contrast enhancement agents in experimental and clinical imaging research. It was approved for clinical use in adult patients in June of 1988. It is an extracellular agent that accumulates in tissue by perfusion dominated processes. Image enhancement improvements using Gd(DTPA) are well documented in a number of applications (Runge et al., Magn, Reson. Imag. 3:85 (1991); Russell et al., AJR 152:813 (1989); Meyer et al., Invest. Radiol. 25:S53 (1990)) including visualizing blood-brain barrier disruptions caused by space occupying lesions and detection of abnormal vascularity. It has recently been applied to the functional mapping of the human visual cortex by defining regional cerebral hemodynamics (Belliveau et al., (1991) 254:719).
Another chelator used in Gd contrast agents is the macrocyclic ligand 1,4,7,10-tetraazacyclododecane-N,N',N"N"'-tetracetic acid (DOTA). The Gd-DOTA complex has been thoroughly studied in laboratory tests involving animals and humans. The complex is conformationally rigid, has an extremely high formation constant (logK=28.5), and at physiological pH possess very slow dissociation kinetics. Recently, the GdDOTA complex was approved as an MRI contrast agent for use in adults and infants in France and has been administered to over 4500 patients.
Another technique for imaging cells, frequently used in developmental biology, uses optical dyes, i.e. photoluminescent compounds, to visualize both subcellular and extracellular structures, as well as developmental cell lineages.
It is an object of the invention to provide bifunctional detection agents that can simultaneously behave as both an optical dye as well as an MRI contrast agent. Such agents can be visualized using either MRI and common optical (photoluminescent) techniques.